net/phy: micrel: Be more const correct

In a few places in this driver, we weren't using const where we could
have. Use const more.

In addition, change the arrays of strings in ksz9031_config_init() to be
not only const, but also static.

Signed-off-by: Jaeden Amero <jaeden.amero@ni.com>
Signed-off-by: David S. Miller <davem@davemloft.net>